Radiolucent molybdenum-containing master alloys

ABSTRACT

The present invention relates to a method for producing a Mo-containing master alloy that is radiolucent. In accordance with the present invention, two elements may be used to reduce the density of a Mo-containing master alloy enough to make the master alloy radiolucent, aluminum or titanium. Aluminum is required in the particular titanium alloy in the same weight ratio as Mo and cannot be used to decrease the master alloy density without skewing the ratio. Since the master alloy is being added to a titanium melt, much more titanium can be used to reduce the master alloy density.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. provisional application Ser. No. 61/782,212 which was filed in the United States Patent and Trademark Office on Mar. 14, 2013.

BACKGROUND OF THE INVENTION

Molybdenum (Mo) is a relatively high-density metallic element. It is used for the production of Ti 6Al 2Sn 4Zr 6Mo and other multi-component Mo-containing titanium alloys. Details related to such alloys are found in U.S. Pat. Nos. 4,104,059 and 4,119,457, herein incorporated by reference. When added uniformly to these alloys, it conveys the benefit of stabilizing the beta titanium grain structure. This structure improves ductility and oxidation resistance of the alloy when used at high temperatures. These Mo-containing titanium alloys are used in the compressors of jet turbine engines.

If the Mo is present in the titanium alloy as a discrete particle, it can act as a low-cycle fatigue crack initiation site that can result in catastrophic engine failure. Mo is therefore identified as a potential High Density Inclusion (HDI). Every Mo-containing master alloy must be certified to be free from HDIs.

Two vacuum melting protocols are accepted by the aerospace industry for premium quality titanium alloys. An older method is called Triple-VAR. This method refers to melting the titanium alloy ingot through three, progressively larger, vacuum arc remelt (VAR) furnaces. A newer approved method allows one cold-hearth melt (such as plasma or electron beam) followed by two VAR melts.

An HDI introduced into this melting process can survive into the final ingot if it is too large to dissolve during the three melting cycles. Jet engine manufacturers have determined that particles of tungsten smaller than about 0.015″ will be effectively removed during the melting process. Particles larger than this must be eliminated before melting by process controls and inspection.

Since the Mo containing master alloys cannot be x-ray inspected to this 0.015″ standard, the practice accepted in the industry is to crush Mo containing master alloys to −20 mesh or 0.033″. This assures that no larger HDI particles could be introduced into the melting process.

Each manufacturer consolidates the raw materials (Ti sponge, master alloy and blended elemental) differently prior to melting. Most of the industry can use −20 mesh master alloys. One titanium ingot producer has a compaction process that is unable to use the −20 mesh master alloy. The −20 mesh master alloy creates a compact that is insufficiently strong and prone to breaking Manufacturers require at least a ¼″×20 mesh size in order to produce compacts that are strong enough to melt.

In order to meet both the requirement for powdered master alloy for defect removal and ¼″ particle sizes for strength, a process of powdering and subsequently remelting the master alloy is a current practice. This meets the existing requirements, although in a very labor intensive and costly way. The cost of powdering, remelting and crushing the old master alloy is expensive. It is also understood that remelting the master alloy does not assure defect removal and may cause HDIs to form during subsequent remelting.

Another method of reducing the apparent density of the master alloy is to powder and reagglomerate the powder using aluminum or titanium as a cement. This is a more expensive process and does not provide a strong master alloy particle. In practice, it has been found that HDIs were formed during the sintering process.

The accepted inspection method for detecting an HDI is automated fluoroscopic inspection. Mo-containing master alloys are too dense to transmit x-rays and cannot be certified by this method. The present invention describes a lower density Mo-containing master alloy that can be inspected with an automated fluoroscope.

Radiolucency is a function of the master alloy density and the thickness or cross-section of the particle being examined. The standard size of master alloy accepted by the titanium melting industry is ¼″×20 mesh. The present invention describes successful inspection of ¼″×20 mesh particles.

SUMMARY OF THE INVENTION

The present invention relates to a method for producing a Mo-containing master alloy that is radiolucent. In accordance with the present invention, two elements may be used to reduce the density of a Mo-containing master alloy enough to make the master alloy radiolucent, aluminum or titanium. Aluminum is required in the particular titanium alloy in the same weight ratio as Mo and cannot be used to decrease the master alloy density without skewing the ratio. Since the master alloy is being added to a titanium melt, much more titanium can be used to reduce the master alloy density.

It has been shown that between 20%-50% added titanium can lower the master alloy density enough to render it radiolucent and therefore suitable for x-ray inspection. This will in turn permit the supply of a ¼″ crushed product that has been melted only once.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An important advantage of the present invention is to reduce the risk of HDIs. This single thermite melt process is also the lowest cost production method; further details of this reaction are found in U.S. Pat. No. 5,769,922, herein incorporated by reference. The low Mo phases that form during crystallization do not have the opportunity to become enriched during a subsequent master alloy melt.

In accordance with the present invention, the Mo-containing master alloy is designed to be radiolucent. The addition of titanium may be used to lower the density of any master alloy too dense to inspect by x-ray. This includes master alloys containing niobium, molybdenum and other high-density elements. The master alloys to which this invention may be applied include at least one high-density element and may also include aluminum (Al), tin (Sn), zirconium (Zr), vanadium (V), iron (Fe), chromium (Cr) and silicon (Si). Aluminothermic reduction of the component oxides is the most efficient method of producing this alloy.

In the testing of the present invention, a composition of AlMoTi suitable for x-ray inspection was produced, which was accomplished by introducing extra titanium to reduce the alloy density.

The ingot was sampled at several locations for chemistry and metallographic analysis. About ¼ of the ingot was cleaned and crushed to ¼″×down. A sample of 500 grams was screened to ¼″×+8 mesh, −8×+20 mesh and −20 mesh. The ¼″×+8 mesh fraction was x-ray inspected using the automated x-ray settings. There were no defects noted during the inspection indicating that the sample was radiolucent. A 0.02″ diameter Pb salt was added to the sample and inspected again. The location of the salt was verified by the x-ray and there were still no indications noted from the alloy. This indicates that the alloy can be x-ray inspected. The x-ray inspection photographs are attached, and results of the experiment follow:

Experimental Results (Wt. %)

Al Mo Ti Fe Si C S O N SAMPLE 1 34.59 42.96 22.04 0.085 0.088 0.020 0.0009 0.039 0.0057 SAMPLE 2 33.61 43.97 22.01 0.084 0.087 0.017 0.0009 0.11 0.0076 SAMPLE 3 34.50 42.30 22.79 0.088 0.087 0.018 0.0009 0.14 0.0073 SAMPLE 4 33.05 43.76 22.78 0.085 0.090 0.016 0.0008 0.32 0.0077 SAMPLE 5 33.27 43.79 22.53 0.090 0.087 0.018 0.0009 0.17 0.0066 SAMPLE 6 35.42 41.06 23.11 0.089 0.088 0.019 0.0008 0.18 0.0061 SAMPLE 7 33.77 43.35 22.47 0.086 0.089 0.025 0.0009 0.26 0.0085

Described below is a 6-2-4-6 master alloy with 10% titanium added. It is less radiolucent than an AlMo alloy with 22% Ti, and is radiolucent at 0.20″ thick but not at 0.25″ thick. It can still be inspected, but at a slightly smaller size.

6-2-4-6 Master Alloy with 10% Titanium Added

Al Mo Sn Ti Zr Wt % 28.05 29.40 10.19 9.93 21.91

Four samples of this alloy were crushed, measured and oriented for x-ray inspection. Samples with increasing thickness were selected and oriented by taping the sample on a card and encasing the card in a plastic sample bag. The cross sections selected were 0.085″, 0.130″, 0.20″ and 0.25″ respectively. The automated fluoroscope was set on the standard setting for 15Al/85V, the highest standard setting, and the samples were inspected by x-ray.

The sample with 0.25″ thickness was insufficiently radiolucent and was detected by the x-ray as a high density particle. All of the thinner sections passed inspection. FIG. 1 shows the 0.20″ sample and an x-ray calibration “salt.” The salt has been marked by the x-ray as a high density particle and the 0.20″ master alloy has not been marked as high density. FIG. 2 is a photograph of the radiolucent AlMo with nominally 22% Ti.

While the present invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications, which are within the true spirit and scope of the present invention. 

What is claimed is:
 1. A method for reducing the density of a molybdenum-containing master alloy which comprises adding to said alloy an amount effective for the purpose of titanium, wherein the master alloy is rendered transparent to x-rays.
 2. The method as recited in claim 1, wherein the alloy is in a form of an ingot.
 3. The method as recited in claim 2, wherein the ingot has a weight of about 120 pounds.
 4. The method as recited in claim 2, wherein the ingot has a weight of about 100 pounds.
 5. The method as recited in claim 1, wherein the alloy is used for disks, blades and seals of turbine engines.
 6. The method as recited in claim 1, wherein the alloy is used for airframe parts.
 7. The method as recited in claim 1, wherein the composition is produced by melting pure titanium with the alloy. 